Modern network communication systems are generally of either the wired or wireless type. Wireless networks enable communications between two or more nodes using any number of different techniques. Wireless networks rely on different technologies to transport information from one place to another. Several examples, include, for example, networks based on radio frequency (RF), infrared, optical, etc. Wired networks may be constructed using any of several existing technologies, including metallic twisted pair, coaxial, optical fiber, etc.
Communications in a wired network typically occurs between two communication transceivers over a length of cable making up the communications channel. Each communications transceiver comprises a transmitter and receiver components. The receiver component typically comprises one or more cancellers. Several examples of the type of cancellers typically implemented in Ethernet transceivers, especially gigabit Ethernet transceivers include, echo cancellers, near-end crosstalk (NEXT) cancellers, far-end crosstalk cancellers (FEXT), etc.
A typical wired communications link is shown FIG. 1. The link, generally referenced 10, comprises communications transceivers 12, 20 connected by a channel 18. The communications may comprise, for example Ethernet transceiver (e.g., 10Base-T, 100Base-TX or 1000Base-T). Transceiver 12 comprises a transmitter and receiver which include isolation transformers for coupling the Ethernet signal to the channel. The isolation transformers are effectively high pass filters (HPFs) as represented by the TX HPF block 14 and RX HPF block 16. Similarly, the magnetics in transceiver 20 is represented as RX HPF block 22 and TX HPF block 24. In operation, the transmitter on each end of the connection takes its respective input data and converts and encodes it for transmission over the twisted pair wiring of the channel. Each receiver is optimized to receive the transmitted signal and decode it to generate the received output data.
Since both the channel model and the high pass filters are linear, the order of the components of FIG. 1 can be changed without affecting the output. A simplified block diagram illustrating a combined representation of the transmitter and receiver high pass filters used to model the baseline wander impairment is shown in FIG. 2. The combined representation, generally referenced 30, comprises a channel 32 having an input αk and an output zk and a combined TX/RX HPF 34 having an output yk.
Ethernet transceivers on either end of a link are AC coupled to the twisted pair wiring connecting them to each other. Most communication networks (including Ethernet networks) whose links are AC coupled suffer from what is referred to as baseline wander or DC droop. For example, wired Ethernet links such as 10, 100 or 1000 Mbps links all exhibit baseline wander. Baseline wander occurs when a very long pulse propagates through an isolation transformer. Decoupling transformers are a standard component in Ethernet receiver circuits. Decoupling transformers act as a high-pass filter having very low cutoff frequencies which typically prevents most frequencies less than 4 kHz from passing through to the receiver circuit. The decoupling transformer, acting as a high pass filter with an extremely low cutoff frequency, eliminates the DC component of the incoming waveform and causes a long pulse to drift towards the common mode. This is known in the art as “DC droop.” Thus, the baseline wander is created as a result of the high pass frequency response of the magnetics in both the transmitter and the receiver.
As a result, transmitted pulses are distorted by a droop effect similar to the exaggerated example shown in FIG. 3. In this example, the transmitted symbol 40 is a pulse amplitude modulated (PAM) signal. The received symbol 42 exhibits the droop effect due to the effect of the magnetics (i.e. high pass filter) in the signal path. It is noted that in long strings of identical symbols, the droop can become so severe that the voltage level passes through the decision threshold, resulting in erroneous sampled values for the affected pulses.
In addition to the baseline wander caused by the far end signal, in communication systems using bi-directional transmission on the same line (such as gigabit Ethernet), the echo signal also contributes its own baseline wander in the form of undesired low frequency hybrid compensation as shown in FIG. 4. The circuit, generally referenced 50, comprises a hybrid 56, amplifiers 52, 58, TX high pass filter 54 and adder 60. Note that unlike the far end baseline wander, the echo baseline wander is generated by a single high pass filter. Note also that the echo signal does not suffer from channel attenuation. In fact, it can be up to 10 dB stronger then the far end signal, which makes the echo baseline wander a substantial source of interference.
When the secondary winding of the decoupling transformer decouples the received waveform and sends the signal to the transceiver chip, the DC component of the original waveform does not pass through. When a coded signal (e.g., MLT-3 coded signal) remains constant (i.e. there are no transitions) for periods longer than the cut-off frequency of decoupling transformer, the output of decoupling transformer begins to decay to common mode. This phenomenon is caused by the inductive exponential decay of the decoupling transformer.
Depending on the particular code used, certain strings of bits will generate more baseline wander than others. For example, since the MLT-3 code has a transition for every 1 bit and no transition for every 0 bit, only constant 0 bits (not constant 1 bits) converted into MLT-3 code produce a baseline wander condition. Multiple baseline wander events result in an accumulation of offset which manifests itself either more at +1 V or more at −1 V, depending on the direction the wander goes over time. While certain data patterns can cause very severe baseline wander, statistically random data can reduce the amount of baseline wander, but it would still be significant.
The effects of baseline wander can be reduced, however, by encoding the outgoing signal before transmission. This also reduces the possibility of transmission errors. The early Ethernet implementations, including 10Base-T, used the Manchester encoding method wherein each pulse is identified by the direction of the midpulse transition rather than by its sampled level value.
A problem with Manchester encoding, however, it that it introduces frequency related problems that make it unsuitable for use at higher data rates. Ethernet versions subsequent to 10Base-T all use different encoding procedures that make use of one or more of the techniques of data scrambling, expanded code space and forward error correcting codes.
Data scrambling is a technique that scrambles the bits in each byte in an orderly and recoverable way. Some 0s are changed to is, some is are changed to 0s, and some bits are left the unchanged. The result is reduced run-lengths of same-value bits, increased transition density and easier clock recovery. Expanding the code space is a technique that allows the assignment of separate codes for data and control symbols (e.g., start-of-stream and end-of-stream delimiters, extension bits, etc.) which assists in the detection of transmission errors.
Even after coding and scrambling, baseline wander can still occur depending on the case and input data. For example, in 100Base-TX baseline wander can still occur because numerous runs of 0 bits can be generated by the scrambler. The scrambler generates numerous 0 bits when certain packets, known as “killer packets,” enter the scrambler. The probability of a killer packet entering a scrambler is a small number out of all the possible data packet permutations. Further, even if a killer packet enters the scrambler, a problem arises only if the data pattern aligns with the scrambler seed. The probability of this happening is one out of every 2,047 tries. Although the occurrence of killer packets are a rare occurrence in the real world statistically, they are often used in during the design and testing of transceivers to demonstrate the baseline wander problem.
Forward error correcting codes are encodings which add redundant information to the transmitted data stream so that some types of transmission errors can be corrected during frame reception. Forward error-correcting codes are used in 1000Base-T to achieve an effective reduction in the bit error rate. Ethernet protocol limits error handling to detection of bit errors in the received frame. Recovery of frames received with uncorrectable errors or missing frames is the responsibility of higher layers in the protocol stack.
Therefore, what is needed is an apparatus and method that is effective in mitigating the effects associated with baseline wander. Ideally, the mechanism would have minimal cost impact in terms of components, power consumption, computing resources and board or chip real estate.